This invention relates to pressure-sensitive adhesives and sheet material, including tapes and the like, coated therewith.
Three properties which are useful in characterizing the nature of pressure-sensitive adhesives are tack, peel strength, and shear strength. The first measures the adhesive's ability to adhere quickly, the second its ability to resist removal by peeling back on itself from a surface, and the third its ability to hold in position when shearing forces are exerted. Generally speaking, the first two are directly related to each other but are inversely related to the third, cf., e.g., Sanderson U.S. Pat. No. 4,077,926, Col. 1, lines 31-36. For example, although a given adhesive may exhibit high tack and high peel strength, the shear strength may be unacceptable for the intended purpose. Conversely, while an adhesive may exhibit high shear strength, the tack and peel adhesion may be unacceptable for the intended purpose. These shortcomings are often exacerbated at extremes of high and low temperature.
Modern industry presents a number of situations where pressure-sensitive adhesive products having balanced properties (i.e., excellent tack, peel strength, and shear strength) would be highly desirable. Some examples of these situations include installation of coated abrasive on power tools at low temperatures, installation of adhesive-coated substrates, e.g. tapes, at low temperatures. In these same situations, it is often necessary to have high peel strength at high temperatures.
Over the past several years rising energy costs have prompted occupants of buildings to conserve energy by means of insulation. The installation of shrinkable film or shrinkable/stretchable film over windows by means of double-stick tape (tape bearing a layer of adhesive on both major surfaces thereof) adhered to the interior window frame and to the film is known to exert a constant force at the film-adhesive interface and provide a degree of insulation. Normally the film is located between the window pane and the interior of the structure. Under normal room temperature conditions, the adhesive has physical properties which allow it to maintain the bond. There are many reasons for installing insulating film on the exterior part of the window. Some users may not want tape on their interior walls or interior window frames, they may want ready access to their windows, or they may have ornate window sills or frames which would hinder installation of the film on the inside. If the film is installed outdoors, the adhesive will be subjected to high temperatures during summer months and low temperatures during winter months. Of particular concern is the fact that, at elevated temperatures, the peel strength and shear strength of conventional pressure sensitive adhesives is greatly reduced. At the same time the film is exerting shrink tensions as high as 510 psi causing the bond to fail and the film to pull away from the exterior window frame. At reduced temperatures conventional pressure-sensitive adhesives have greatly reduced tack properties, making it difficult to create an adhesive bond with the film or window frame.
Rubber based adhesives, which are normally used for installation of shrinkable film currently commercially available, exhibit poor weathering characteristics, loss of tack at low temperatures, e.g. 0.degree. C., and loss of shear at high temperatures, e.g. 65.degree. C.
Thus, there is a need for pressure-sensitive adhesives which have balanced adhesive properties over a wide temperature range, good adhesion to plastic films which for the most part have low energy surfaces and which can be prepared easily and economically on conventional equipment.